1. Field of the Invention
This invention relates to an optical head wherein reflected light of a laser beam from a record medium is condensed into a beam of light and then split into two beams of light which are received by a light receiving element to produce a servo signal and/or a photomagnetic reproduction signal, and more particularly to an optical head suitable for use with a photomagnetic recording/reproducing device.
2. Description of the Prior Art
An exemplary one of conventional optical heads of the type mentioned is first described with reference to FIG. 17. The optical head shown includes a semiconductor laser 11 serving as a source of a laser beam, a collimator lens 12 for collimating light from the semiconductor laser 11 into substantially parallel light, a separating element 13 such as a separating prism for reflecting light from the light source 11 but transmitting reflected light from a photomagnetic disk 15 which serves as a record medium, an objective lens 14 for focusing light reflected from the separating element 13 upon a face of the photomagnetic disk 15, a cylindrical lens 16 having a focusing function only in one direction, a polarizing beam splitter 17 for splitting a beam of light from the cylindrical lens 16 into P polarized light and S polarized light, and first and second light receiving elements 18 and 19 for receiving beams of P polarized light and S polarized light, respectively, from the polarizing beam splitter 17 and individually converting the received light signals into electric signals.
In the optical head, a beam of laser light emitted from the semiconductor laser 11 is substantially collimated by the collimator lens 12, then reflected by the separating element 13, and then focused to a diffraction limit upon the photomagnetic disk 15 by the objective lens 14.
Then, light reflected from the photomagnetic disk 15 is converted into a converging beam of light (a beam of light which converges to a point as it advances) by the objective lens 14 and then passes through the separating element 13, whereafter it passes through the cylindrical lens 16 toward the polarizing beam splitter 17 whereupon astigmatism is provided to the beam of light. Then, P polarized light which has passed through the polarizing beam splitter 17 forms a spot of light on a light receiving face of the first light receiving element 18 while S polarized light reflected by the polarizing beam splitter 17 forms a spot of light on a light receiving face of the second light receiving element 19.
In a photomagnetic recording/reproducing system which employs such an optical head as described above, information signals are recorded depending upon directions of magnetization of a perpendicular magnetic film of a photomagnetic disk, and the direction of rotation of linearly polarized light of a beam of laser light reflected by the disk is detected to reproduce the record signals. In particular, a record signal will appear as a difference in amount of light between beams of P polarized light and S polarized light split by the polarizing beam splitter 17, and consequently, a photomagnetic reproduction signal is differentially detected depending upon a difference between outputs of the first and second light receiving elements 18 and 19 individually produced in accordance with amounts of light received.
Meanwhile, a focus error signal and a tracking error signal (Servo signals) are produced from outputs of the light receiving elements 18 and 19. In the arrangement described above, a focus error signal is produced in accordance with an astigmatism method.
With such a conventional optical head as described above, however, a cylindrical lens or the like for providing astigmatisms to light is required. Futhermore, two light receiving elements must be used because P polarized light and S polarized light are split at a right angle (by splitter 17). Consequently, the conventional optical head requires a comparatively great number of components and a high degree of accuracy in positional relationship between a light path and such components. Accordingly, the optical head is disadvantageous in adjusting operation and reliability. Further, the overall size of the optical head itself is comparatively great because light paths of beams of P polarized light and S polarized light make the right angle. Since the polarizing beam splitter 17 has an angle dependency in polarizing splitting action thereof (a difference between incidence angles of two beams of light makes a difference between optical paths of the beams of light as seen from FIG. 18), splitting of a beam of light into two beams of P polarized light and S polarized light is not performed in equal conditions. Accordingly, there is a problem that, even if a photomagnetic reproduction signal is detected differentially, some deterioration in the common-mode rejection ratio cannot be eliminated.
In addition, the two light receiving elements 18 and 19 of the conventional optical head described above must necessarily be adjusted in three axial directions because split beams of P polarized light and S polarized light are focused at different points spaced from each other and thus received by the two light receiving elements 18 and 19, respectively, at such locations. Accordingly, the conventional optical head is disadvantageous in adjusting operation and reliability